Deoxidizing nickel base and cobalt base alloys

ABSTRACT

A method of deoxidizing metal while maintaining the carbon content of the metal at a level about equal to or lower than the level prior to deoxidizing. It comprises the steps of introducing hydrocarbon deoxidizer and diluent gas into a vessel containing molten metal, determining the effect of the hydrocarbon deoxidizer and diluent gas upon the carbon content of the metal and controlling the proportion of hydrocarbon deoxidizer to diluent gas so that the average rate of carbon leaving the vessel is about equal to or greater than the average rate of carbon being introduced into the vessel.

United States Patent [191 Ramachandran 1*June 11, 1974 DEOXIDIZING NICKEL BASE AND COBALT BASE ALLOYS Notice: The portion of the term of this patent subsequent to Apr. 3, 1990, has been disclaimed.

[22] Filed: Nov. 24, 1972 [21] Appl. No.: 307,661

Related US. Application Data [62] Division of Ser. No. 75,738, Sept. 25, 1970, Pat. No.

[52] US. Cl 75/82, 75/59, 75/60 [51] Int. Cl C22b 23/00 [58] Field Of Search 75/82, 93, 60, 59, 49; v 72/82, 59, 60

i ,[56] References Cited UNITED STATES PATENTS 3,2l8,l56 11/1965 Sluis et al. 75/49 3,725,04l 4/1973 Ramachandran 75/60 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-M. J. Andrews Attorney, Agent, or Firm-Vincent G. Gioia; Robert F. Dropkin [5 7] ABSTRACT A method of deoxidizing metal while maintaining the carbon content of the metal at a level about equal to or lower than the level prior to deoxidizing. lt comprises the steps of introducing hydrocarbon deoxidizer and diluent gas into a vessel containing molten metal, determining the effect of the hydrocarbon deoxidizer and diluent gas upon the carbon content of the metal and controlling the proportion of hydrocarbon deoxidizer to diluent gas so that the average rate of carbon leaving the vessel is about equal to or greater than the average rate of carbon being introduced into the vessel.

10 Claims, No Drawings naoxrmznvc NICKEL BASE AND coBAL'r BAS ALLOYS desirable to lower the oxygen content of a melt since oxygen dissolved in a melt can precipitate as nonmetallic inclusions which adversely affect the properties of the metal.

Various deoxidizing methods have been employed in the past. One method involves the use of highly reactive elements; e.g., silicon, titanium and/or aluminum, which combine with oxygen in the melt to form oxides that subsequently separate from the melt. Sufficient time must, however, be provided for the oxides and melt to separate. A secondmethod involves the mixing of a predetermined quantity of carbon with the melt and a dynamic hydrogen atmosphere tocontrol carbon boil. This method is effective for lowering the oxygen content but often adversely affects the desired low carbon content. It is disclosed in U.'S. Pat. No. 3,188,198 which issued on June 8, 1965. A third method, particularly effective for alloy steels with high carbon contents, involves the lowering of the partial pressureof 1 carbon monoxide in the vessel. A lowering of the partial pressure of carbon monoxide changes equilibrium relationships and shifts the attainable end point carbon to lower levels without necessitatingexcessive oxidation of metallic components and, thereby, frees carbon to combine with oxygen within the melt. Reduction in the partial pressure can be accomplished by reducing the pressure in the vessel and/or by introducing argon into'the vessel. This third method, however, does not measure up to theoretical expectations as the carbonoxygen reaction fails to proceed to completion. From a thermodynamic point of view, the system acts as if it were under a higher pressure.

Another prior art process of considerable interest is disclosedin an article entitled Deoxidation Techniques for Vacuum-Induction Melting by W. F. Moore. It appeared on pages 918- 921 in theDecember 1963 issue of the Journal of Metals. The process described therein employednatural gas which consisted of, by volume, 94.9 percent CH 3.2 percentC H 1.0 percent C l-l 0.l percentC H. 0.6 percent CO 0.1 percent H and 0.1 percent N, CO, to deoxidize a melt. Results from the process were promising with regard to the degree of deoxidizing but were unfortunately disappointing to processors who require both low carbon and oxygen contents intheir steel. The natural gas injected an excess of carbon into the melt and thereby, raised its carbon content.

it would appearthat the major shortcoming of the process described inthe above referred to Journal of Metals article could be rectified by reducing the amount of natural gas injected into themelt. The most obvious manner of accomplishing this would be to simply reduce the natural gas input flow rate. This however,-diminishesthe mixing caused by the input of the gas which in turn reduces the degree of reaction between carbon that evolves from the gas and oxygen in the melt.

The present invention provides a method which effectively deoxidizes a melt'while controlling the car-bon content at a level about equal to or lower than that which was present prior to deoxidizing. It employs at least one. hydrocarbon as a deoxidizer and at least one diluent gas. The diluent gas enables the processor to use small amounts of hydrocarbon deoxidizer, thereby precluding excessive injection of carbon into the melt, while maintaining a gaseous injection rate which is sufficient to insure adequate mixing between the hydrocarbon deoxidizer and the melt. Thus, the hydrocarbon deoxidizer input rate can be lowered without reducing the rate of oxygen reaction with carbon, by introducing a diluent gas with the hydrocarbon deoxidizer.

It is accordingly an object of this invention to provide a method of deoxidizing molten metal.

It is an additional object of this invention to provide a method of deoxidizing molten metal while maintaining thecarbon content of the metal at a level about equal to or lower than the level prior to deoxidizing.

The present invention comprises the steps of introducing hydrocarbon deoxidizer and diluent gas into a vessel containing molten metal, determining the effect of the hydrocarbon deoxidizer and diluent gas upon the carbon content of the metal and controlling the proportion of hydrocarbon deoxidizer to diluent gas so that the average rate of carbon leaving the vessel is about equal to or greater than the average rate of carbon being introduced into the vessel.

The invention embraces the use of one or more hydrocarbon deoxidizers chosen from a wide spectrum of gaseous and liquid hydrocarbons and hydrocarbon containing substances as well as the use of one or more diluent gases chosen from a wide spectrum of diluent gases. Illustrative hydrocarbons and hydrocarbon containing substances are methane, ethane, propane, ethylene, water gas and natural gas. Illustrative diluent gases are argon, nitrogen, hydrogen and carbon monoxide. Liquid hydrocarbons and hydrocarbon containing substances require the additional step of atomizing the liquid into the diluent gas stream. The hydrocarbon deoxidizer and diluent gas can be blown into or blown onto the top of the melt.

The effect of the hydrocarbon deoxidizer upon the carbon content of a melt can be determined from the initial and final melt analysis of a previously deoxidized melt. A comparison of the initial and final analysis along with the input rate and analysis of the hydrocarbon deoxidizer and/or diluent gas (diluentgas is not necessary here as this is not necessarily a heat requiring a low carbon level) introduced into the-vessel sets forth the information necessary to determine the efficiency at which carbomfrom the deoxidizer, combined with oxygen. The efficiency .at which carbon and oxygen combined tells a processor what the proportion of hydrocarbon deoxidizerto diluent gas should be for subsequent heats, of similar chemistry, which are to be deoxidized under similar conditions, e.g., similar gaseous injection rates. For example, a heat deoxidized with 80 percent hydrocarbon deoxidizer and 20 percent diluent gas and having a percent carbon-oxygen reaction efficiency indicates that subsequent heats should use 40 percent or less hydrocarbon deoxidizer and percent or more diluent gas if the heats are of similar chemistry and are to be similarly deoxidized.

Although the above described procedure for determining the effect of the hydrocarbon deoxodizer is far better than adequate, it does have a shortcoming. The end-point control may not be too precise due to heat variations in analysis, temperature and efficiency of gas blowing.

An alternative process for determining the effect of the hydrocarbon deoxidizer overcomes the shortcoming of the above described procedure. It involves calculating the rate at which carbon is introduced to the vessel and the rate at which it leaves the vessel. The carbon input rate can be calculated from the analysis and input rate of hydrocarbon deoxidizer and diluent gas. The carbon output rate can be calculated from the output rate and analysis of the gases exiting the vessel (a monitoring system can be used to analyze the exiting gases). The calculations enable a processor to control the carbon content of the melt by adjusting the rate at which carbon is introduced to the vessel.

The following paragraphs are exemplary of the type of reactions which occur during the deoxidizing method of this invention and of method for calculating both the carbon input and output. Methane, CH has been chosen as the hydrocarbon deoxidizer for purposes of illustration.

sures which are at, below or above atmospheric pressure.

The reaction which leads to carburization of the bath I CH (gas) C 2H (gas) An additional carburization reaction which can occur to an intolerable degree when there is an excessive amount of hydrocarbon deoxidizer 1s:

CH (gas) c 2H (gas) The hydrocarbon deoxidizer cracks and deposits carbon (soot) in the cooler parts of the vessel.

The carbon input and input rate can be obtained from the following equations:

poundsof carbon: total cu. ft. X .1 mole introduced into of methane 360 ft the vessel introduced into the vessel X atomic wt. of carbon (lbs.)

1 mole rate carbon is being pounds of carbon introduced into =introduced into the vessel the vessel time The carbon output and output rate can be obtained from the following equations:

total cu. ft. of pounds of CH4. CO- and 1 mole X atomic wt'. of carbon (lbs.)

1 mole rate of carbon pounds of carbon I leaving the vessel leaving the vessel time No numerical value can be set for the ratio of hydrocarbon deoxidizer to diluent gas as it can change throughout the deoxidizing treatment. At times, it is even desirable to complete the final stages of deoxidation with an inert gas and without any hydrocarbon deoxidizer. The inert gas will lower the partial pressure of carbon monoxide, change equilibrium relationships and shift the attainable end point carbon to lower levels without necessitating excessive oxidation of metallic components, thereby freeing carbon to combine with oxygen within the melt. It will additionally cause a mixing of the melt which will promote the carbon-oxygen reaction. As a general rule the hydrocarbon deoxidizer and the diluent gas are injected into the vessel at an average gaseous injection rate of at least 20 cu.ft. per hour. A preferred average gaseous injection rate is at least 30 cu.ft. per hour. Lower injection rates are, however, embraced within this invention. A precise value can not be set for the minimum rate as it fluctuates with process variables, such as the depth of the molten metal.

The invention can additionally encompass controlling of the final oxygen content. This entails knowledge of the oxygen content prior to or at some stage during deoxidation and calculating of the amount of oxygen leaving the system. Oxygen in the melt can be measured by an EMF cell or by chemical analysis. The amount of oxygen leaving the system can be calculated from an analysis of the gases exiting from the system and from the following equations:

pounds of oxygen 7 total cu.ft. of leaving the system- CD, CO and H 0 v leaving? the system 1 mole 360 cu. ft.

atomic wt. of oxygen (lbs) average molecular wt. of gases leavmg the vessel rate of oxygen pounds of oxygen ime leavmg the system leavmg the system t Knowing the measured oxygen in the melt and the rate at which oxygen is leaving the system enables a processor to determine the amount of oxygen remaining in the tion is however, applicable to a wide range of metals which include both nickel and cobalt base alloys.

Three stainless steel heats (A, B and C) having the carbon, oxygen and nitrogen analysis shown below in Table l were deoxidized.

TABLE I uz,4/ l 6 Analysis (ppm) Heat C O N is l 140 14s Heats A and B were deoxidized for 30 minutes with pure methane and heat C was deoxidized for 30 minutes with a gaseous mixture comprised of 2 percent methane and 98 percent argon (diluent gas). The average input rate for both the pure methane used in conjection with heats A and B and for the methane-argon mixture of heat C was 36 cu.ft. per hour.

The carbon, oxygen and nitrogen analysis for the deoxidized heats is shown below in Table II.

TABLE ll 1.4/16 Analysis (ppm) Heat C O N A B C It will be apparent to those skilled in the art that the novel principles of .the invention disclosed herein in connection with specific examples thereof will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific examples of the invention described herein.

I claim:

1. A method of deoxidizing molten metal from the group consisting of nickel base and cobalt base alloys and controlling its final oxygen content while maintaining a carbon level about equal to or lower than the carbon level which was present prior to deoxidizing, which comprises the steps of: analyzing molten metal from the group consisting of nickel base and cobalt base alloys to determine its oxygen content; introducing hydrocarbon deoxidizer and diluent gas at an average injection rate of at least 20 cu.ft. per hour into a vessel containing said molten metal at a subatmospheric pressure. said hydrocarbon deoxidizer reacting with oxygen within said metal to form gaseous carbon compounds which exit from said vessel; determining the effect of the hydrocarbon deoxidizer and diluent gas upon the carbon content of said metal; controlling the proportion of hydrocarbon deoxidizer to diluent gas so that the average rate of carbon leaving said vessel is about equal to or greater than the average rate of carbon being introduced into said vessel; and discontinuing said introduction of hydrocarbon deoxidizer and diluent gas into said vessel after a predetermined amount of oxygen has been removed from said metal.

2. A method according to claim 1 wherein said introducing of hydrocarbon deoxidizer and diluent gas into said vessel containing molten metal comprises the step of blowing hydrocarbon deoxidizer and diluent gas into said molten metal.

3. A method according to claim 1 wherein said introducing of hydrocarbon deoxidizer and diluent gas into said vessel containing molten metal comprises the step of blowing hydrocarbon deoxidizer and diluent gas onto said molten metal.

4. A method according to claim 1 wherein said molten metal is a second melt and wherein said determining of the effect of said hydrocarbon deoxidizer and diluent gas upon the carbon content of said melt, comprises the steps of: analyzing a first melt contained within a vessel to determine its carbon content; deoxidizing said first melt with hydrocarbon deoxidizer; calculating the amount of carbon introduced into said vessel containing said first melt; analyzing said deoxidized first melt to determine its carbon content; and calculating the efficiency of said introduced carbon in deoxidizing said first melt.

5. A method according to claim 1 wherein said determining of the effect of said hydrocarbon deoxidizer and diluent gas upon the carbon content of said metal comprises the steps of: calculating the rate at which carbon is introduced into said vessel; and calculating the rate at which carbon leaves said vessel.

6. A method according to claim 5 including the step of analyzing the gases exiting from said vessel.

7. A method according to claim 1 wherein said average injection rate is at least 30' cu.ft. per hour.

8. A method according to claim 1 wherein said hydrocarbon deoxidizer is comprised of methane.

9. A method according to claim 8 wherein said diluent gas is comprised of argon.

10. A method according to claim 1 wherein said hydrocarbon deoxidizer is a liquid and including the steps of atomizing and mixing said liquid hydrocarbon deoxidizer into said diluent gas. 

2. A method according to claim 1 wherein said introducing of hydrocarbon deoxidizer and diluent gas into said vessel containing molten metal comprises the step of blowing hydrocarbon deoxidizer and diluent gas into said molten metal.
 3. A method according to claim 1 wherein said intRoducing of hydrocarbon deoxidizer and diluent gas into said vessel containing molten metal comprises the step of blowing hydrocarbon deoxidizer and diluent gas onto said molten metal.
 4. A method according to claim 1 wherein said molten metal is a second melt and wherein said determining of the effect of said hydrocarbon deoxidizer and diluent gas upon the carbon content of said melt, comprises the steps of: analyzing a first melt contained within a vessel to determine its carbon content; deoxidizing said first melt with hydrocarbon deoxidizer; calculating the amount of carbon introduced into said vessel containing said first melt; analyzing said deoxidized first melt to determine its carbon content; and calculating the efficiency of said introduced carbon in deoxidizing said first melt.
 5. A method according to claim 1 wherein said determining of the effect of said hydrocarbon deoxidizer and diluent gas upon the carbon content of said metal comprises the steps of: calculating the rate at which carbon is introduced into said vessel; and calculating the rate at which carbon leaves said vessel.
 6. A method according to claim 5 including the step of analyzing the gases exiting from said vessel.
 7. A method according to claim 1 wherein said average injection rate is at least 30 cu.ft. per hour.
 8. A method according to claim 1 wherein said hydrocarbon deoxidizer is comprised of methane.
 9. A method according to claim 8 wherein said diluent gas is comprised of argon.
 10. A method according to claim 1 wherein said hydrocarbon deoxidizer is a liquid and including the steps of atomizing and mixing said liquid hydrocarbon deoxidizer into said diluent gas. 